1. Field of the Invention and Related Art Statements
The present invention relates to a multilayered ceramic substrate fireable at low temperatures.
Recently, there are various requirements for a multilayered ceramic substrate, such as lower signal propagation delay time, higher wiring density and installationability of a large chip, and thus it is necessary to achieve a low dielectric constant, a low electric resistivity and a low thermal expansion coefficient. To this end, use of a multilayered ceramic substrate fireable at low temperatures is gradually increased.
In the low temperature fireable multilayered ceramic substrates, a metal member is brazed to a ceramic substrate fireable at the low temperature through a metalizing layer so as to increase an adhesion strength between the ceramic substrate and the metal member.
In this case, a thermal expansion coefficient of the ceramic substrate fireable at low temperatures is generally low. Therefore, if use is made of a metal member made of a Kovar or an Fe-42Ni alloy (both of them having a large thermal expansion coefficient such as .alpha.: about 10.times.10.sup.-6 /.degree.C. at RT.about.800.degree. C.), there occurs a drawback such that the adhesion strength between the metal member and the ceramic substrate is decreased due to a residual stress generated at a connection boundary between the metal member, and largely different thermal expansion coefficients between the ceramic substrate and metal member.
To eliminate the drawback mentioned above, in the Japanese Patent Laid-open Publication No. 63-44751, there is disclosed a technique such that, when a metal lead is brazed at a high temperature and connected to a ceramic substrate fireable at low temperatures, a copper member is arranged as a buffer member between the metal lead and the ceramic substrate.
However, even in the technique disclosed in Japanese Patent Laid-open Publication No. 63-44751, a large and sufficient adhesion strength between the metal lead and the ceramic substrate can not be obtained and thus the above drawbacks still remain.
Further, the low temperature fireable ceramic substrate has such advantages that the dielectric constant of the substrate can be made lower by suitably selecting glass compositions of a glass-ceramic composite body or a crystalline glass body. Therefore, in the multilayered wiring ceramic substrate, an increase of the signal propagation speed and an increase of the wiring density can be realized.
However, since a flexural strength of the low temperature fireable ceramic substrate is low, a sufficient adhesion strength between the metal member and the ceramic substrate can not be obtained, when the metal member for input/output terminals is brazed at a high temperature and connected to the ceramic substrate through the metalizing layer as is the same as the known method. This is because a residual stress is generated at the boundary region between the metal member and the ceramic substrate due to thermal stress on a high temperature soldering using for example an eutectic Ag solder, and thus a generation of micro cracks, in an extreme case, a breakage of the ceramic substrate occurs.
In order to eliminate the drawbacks mentioned above, the flexural strength of the ceramic substrate might be increased. To this end, when use is made of the glass-ceramic composite body as the low temperature fireable ceramic substrate, the following two methods have been known.
First, there is known a method wherein an amount of alumina included in the low temperature fireable ceramic substrate as a ceramic component is increased so as to increase the flexural strength of the ceramic substrate. Secondly, there is known a method wherein use is made of a leaded borosilicate glass as a glass component of the low temperature fireable ceramic substrate, and the glass component is crystallized so as to increase the flexural strength.
However, if the flexural strength of the ceramic substrate is increased by the above two methods, the flexural strength is increased, but a dielectric constant of the ceramic substrate becomes higher correspondingly. Therefore, there is a drawback that the advantage of the low temperature fireable ceramic substrate is reduced.
In order to eliminate the drawbacks mentioned above, there is known a technique in which an adhesion strength is made larger, in Japanese Patent Laid-open Publication No. 62-25497 and Japanese Patent Laid-open Publication No. 62-16598. That is to say, in the Japanese Patent Laid-open Publication No. 62-16598, a green sheet whose flexural strength after firing is larger than that of the ceramic substrate is arranged to the portion on which the metal terminal is connected so as to increase the adhesion strength. Further, in the Japanese Patent Laid-open Publication No. 62-25497, an insulation paste is fired with the metal terminal so as to increase the adhesion strength.
However, in the techniques disclosed in the above publications, since the green sheet or the insulation paste is not co-fired with the ceramic substrate, the adhesion strength is still not sufficient.
Further, in U.S. Pat. No. 4,695,517, there is disclosed a technique such that a first layer including W or Mo and a second layer including only W or Mo are co-fired with an AlN substrate so as to increase an adhesion strength between the metalizing layer and the ceramic substrate.
However, since use is made of W or Mo whose melting point is high, the technique disclosed in U.S. Pat. No. 4,695,517 is not directly applied to the present invention in which use is made of a low temperature fireable ceramic substrate.